We periodically receive inquiries about the category "amerospores" that appears on reports from other laboratories. We do not use this term because it only represents a morphological category of spores that are produced by many unrelated fungi. Instead, we identify all of the amerospores that we can, even increasing magnification to detect subtle characteristics. When no characteristics are present that allow identification of any spore, we list it as "other dark" or "other colorless". These terms make clear the fact that we have not identified these spores and, importantly, separates these into a small group of spores, enabling better interpretation of the data when comparisons are made between locations. Generally, very few spores fall into the "other" or unidentified category. Our analysts have all received college degrees in biology or a related field, many have advanced degrees, all have received rigorous training, and all have enormous resources available to help with spores that are difficult to identify.
With respect to the amerospores listed by other laboratories, we have no way of knowing what kinds of fungi are represented in this category. Some may be the small thin-walled colorless spores that we recognize as basidiospores or ascospores. These cannot be classed as amerospores, because they are the result of sexual reproduction (see definitions below). Others may be single Penicillium, Aspergillus, or related spores. We identify individual spores of the Penicillium and Aspergillus type, even when chains are not present. In the vast majority of cases, we are able to distinguish the tiny projections that remain after the chains fall apart. Also, most Penicillium and Aspergillus spores are not truly colorless, and many have very minute ornamentation.
On a more general note, conidia (asexual spores produced externally on specialized hyphae) can be grouped in several different ways: by color, by shape and septation, and by their differentiation process. A well-trained analyst will use all three of these groupings to place a spore in the appropriate category. However, color, shape and septation are the most commonly used characteristics for identifying spores without accompanying spore bearing structures. These categories are defined below. Finally, using a broad category such as "amerospores" on a report makes interpretation difficult, especially if a high percentage of the spores present on the sample get put into this category. This is because it is harder to tell if there is an inside source of spores and some important water indicators fall into the definition of amerospore.
Dematiaceous: brown to black from melanin pigments
Moniliaceous: colorless or with some pigment other than melanin
Shape and septation (both Dematiaceous and Moniliaceous spores occur in each of these categories):
amerospore: a non-filamentous spore with no septations and with no projections longer than the spore body (does not include strongly curved spores or very long spores).
dictyospore: a multicellular spore with septations that intersect in more than one plane. These are also called muriform spores.
didymospore: a spore with only two cells (does not include strongly curved spores or very long spores).
helicospore: a strongly curved or spiral spore.
phragmospore: a spore with two or more transverse septa (does not include strongly curved spores or very long spores).
scolecospore: a very long spore (includes both septate and non-septate spores).
The following table classifies some very common conidia by color, shape and septation.
|Cladosporium||Dematiaceous||Amerospore, didymospore, phragmospore|